A machinist friend of mine made an incredible prop balance checker for
me. It consists of a 6" long metal tube connected to a 4" diameter,
1/8" thick disc that has another disc, 1/2" thick attached to it. The
smaller disc is of the diameter that fits snuggly into the bottom of
the prop, and the larger disc butts against the bottom of the prop. A
1" diameter x 1" long fitting is turned on a lathe, and bored 3/4" ID
to within 1/4" of the end, and another bore follows of 5/8" ID to
within 1/8" of the end. This 1/8" closed end is drilled in the center
about .02", and then sliced/turned off. This creates a 1" dia x 1/8"
thick disc that is concentric with the fitting. A second 1/2" long
fitting is made that fits snuggly inside the 1" tube and also has a
..02" hole drilled through it. The tube with the attached discs is
inserted into the back of the prop. A thin piece of cable is threaded
through the disc and first fitting, then the second fitting then down
through the 6" tube and the cable secured on the bottom discs. The
second fitting is pushed down into the 6" tube until it is just above
the fore-aft center of the prop, then the first fitting and its disc is
placed over the top end of the tube. Now the cable is used to suspend
the prop. The turned-off disc on the top of the first fitting acts as
an indicator of out-of-balance of the suspended prop. If the prop is
balanced, the turned-off disc will be concentric with the fitting just
under it. Any out-of-balance will cause the disc indicator to be
off-center. The fitting inside the tube, which holds the cable in the
long tube's center, adjusts the sensitivity. When it is above the
prop's fore-aft CG, the sensitivity is low. Pushing it further and
further down into the tube toward this CG, the sensitivity gets greater
and greater, until when it is below the CG, the prop flops over to one
side. When adjusted for good sensitivity, it will show the unbalance of
a #4 washer at the tip of a 63" diameter prop. The indicator disc has
its maximum deflection in the direction opposite from the unbalance. It
shows unbalance in all directions. Try it; you'll like it! Paul